The present invention relates to antennas employing generally radially extending metal fins and more particularly to such antennas for television broadcasting.
In the design of transmitting antennas for television broadcasting, and especially for high VHF-TV frequencies (e.g., 174-216 mHz) and for UHF-TV frequencies (e.g., 470 mHz. to 890 mHz.), it is generally desired to achieve high gain with an omnidirectional pattern (or a pattern of some other desired shape) in the azimuth plane at minimum cost. For omnidirectional performance, the total or maximum signal strength variation as a function of azimuth plane direction cannot be more than approximately 3db (.+-. 1.5db) in accordance with trade acceptance and FCC standards, and such an antenna pattern is very often required by a broadcasting station providing service to a given region or community.
To achieve a high overall antenna gain, it is the usual practice to vertically stack a large number of driven antenna elements, each one defining a "bay", to typically provide a vertical array extending for 20 to 50 feet, or more. In view of the substantial vertical height thus needed, the supporting structure, typically a metal tower or mast pole, on which the elementary driven antenna elements are supported, will generally be made with a relatively large diameter. That is, the supporting structure will generally have a major transverse dimension greater than one-quarter wavelength, at the transmitted frequency, to provide the strength and rigidity necessary to mechanically support the antenna assembly and maintain stability in the wind and weather conditions to which it may be subjected. However, a relatively large diameter supporting structure, (e.g., of about a quarter wavelength or greater) generally produces a so-called "shadowing" effect on the resulting antenna pattern, and due to this effect a simple, single vertical array of driven antenna elements on such a supporting structure will not ordinarily provide the omnidirectional, or other desired shaped pattern required for a particular broadcasting application.
In U.S. Pat. Nos. 3,587,108 and 3,821,745, of the present inventor, there is disclosed and claimed an antenna assembly structure for achieving an omnidirectional or other desired pattern with a single vertical linear array of elementary antennas mounted on a large diameter structure of any conventional construction, wherein the assembly generally comprises the supporting structure, a driven antenna carried by the supporting structure and defining a radiation axis extending therefrom, and at least one end-fire parasitic director carried by the supporting structure. The end-fire parasitic director, in accordance with preferred embodiments of those patents, has a longitudinal axis and comprises discrete conductive plates disposed in spaced relation along that axis. The discrete conductive plates have their respective major dimensions at least a quarter-wavelength in the plane of the electric field vector of the transmitted signal, and each director is positioned relative to the driven antenna and supporting structure so as to alter the shape of the antenna pattern produced by their combined effects to provide the selected overall antenna pattern for the assembly. One or more of such parasitic directors may be employed, as desired, in combination with the supporting structure and driven elements to produce any number of pattern shapes, from omnidirectional to highly directional, and these are described in detail in the aforementioned patents.
Because of the high gain, and consequent great height, requirements of such antenna assemblies, it is often necessary, as previously indicated, to make the transverse dimension of the support structure, such as the diameter of a supporting mast or pipe, or the equivalent dimension of triangular or other shaped towers, not only greater than a quarter wavelength, but at least one-half wavelength or more. And in particular, it should be noted here that although omnidirectional performance is the most common requirement for broadcast antenna designs, it is generally the most difficult criterion to meet. Antennas requiring directive performance are almost always easier to design. Thus, omnidirectional performance is typically the governing design criterion, and because of shadowing effects, the principal controlling parameter is the support mast size (in wavelengths), regardless of the number of plates employed. The employment of large numbers of plates per director and the potential need for more than two directors per bay considerably worsen the problems of supporting the directors and the problems associated with wind load, in addition to increasing the cost of the antenna assembly.
Accordingly, it is an object of the present invention to provide an improved antenna assembly which may employ a low cost, conventional supporting structure, and which obviates or minimizes the aforementioned problems while maintaining a high gain, omnidirectional or other desired antenna pattern using a linear row of driven elements with one per bay required, even for supporting structures having extremely large transverse dimensions.
In practice, conventional steel pipe is commonly employed for the supporting mast since it is relatively inexpensive and can be flanged with standard flanges, whereas "high strength" steels cannot. Consequently, from practical design considerations, such mast pipes for UHF broadcasting, for example, are commonly required to have diameters of from one-half to three-quarter wavelength, based on existing transmitter tube powers, FCC rules on effective radiated power (ERP), yield strengths of the steel, considerations regarding the power handling characteristics of the coaxial feed lines located inside the pipe, etc.
Although with mast structures of less than one-half wavelength diameter, it may be practical in many antenna designs according to the teachings of the aforementioned patents to use only two plates per end-fire director to achieve an omnidirectional pattern, for mast structures having diameters larger than one-half wavelength a greater number of plates per director is generally required to maintain the same omnidirectional performance. However, it has been found that as the mast diameter is increased, which generally increases the required number of plates per director, undesirably sharp lobes and nulls tend to be formed in the antenna pattern. This increases the criticality of the director positions, and in some cases, requires the use of additional end-fire directors for pattern smoothing. In certain cases the 3db omnidirectional pattern tolerance may not be possible to attain at all with only two end-fire directors per bay.
I have discovered that a similar or superior result can be obtained by replacing the parasitic directors by metal fins attached to the support structure, or forward of the driven element, or both. Using this technique omnidirectional performance within .+-. 1.5 db has been obtained on supports up to 3/4 wavelength diameter and directive patterns of many shapes have also been obtained. This structure is much more sound structurally than one using parasitic directors, presents such less wind load, is less affected by mutual coupling among bays, and by effects of supporting members which parasitic directors required. Further, the fins protrude less than the prior art parasitic directors from the supporting pipe and accordingly interfere less with climbing, and are less subject to the effects of wind, ice and dirt. The antenna of this invention is more broadband, that is to say, it has greater bandwidth for a given input VSWR limitation.
The antenna of this invention utilizes a plurality of generally radially extending fins acting as a surface wave carrier, bringing the energy around the surface of the pipe to fill in, partially or completely as desired, the normal shadow caused by the pipe supporting structure on the pattern of the driven element.
These and other objects, feature and advantages of the invention, will, in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawings which form an integral part thereof.